Electrochemical behaviour and stress corrosion cracking of 70 : 30 copper–nickel alloy in monoethanolamine solutions

Abstract

The electrochemical behaviour and stress corrosion cracking (SCC) susceptibility of 70 : 30 Cu–Ni alloy were investigated in 18% monoethanolamine (MEA) solutions at 60 and 90°C. The MEA solutions were used under the deaerated, aerated and H₂S saturated conditions. Potentiodynamic polarisation experiments showed that the alloy exhibits an active–passive transition in both the deaerated and aerated solutions at pH 9. On the other hand, it underwent active anodic dissolution in the absence of passivity in the same solutions when the pH was increased to 12, and also in the H₂S saturated solution. Open circuit potential measurements showed that the corrosion potentials reached steady state values relatively quickly and that the alloy remained active under all conditions of test. Linear polarisation measurements revealed that the corrosion rates were generally higher in the aerated solutions than in the deaerated ones and tended to be higher in solutions of pH 12 than at pH 9. The corrosion rates were significantly higher in the H₂S saturated solution than in the other two solutions. The 70 : 30 Cu–Ni alloy was found to be susceptible to transgranular SCC in the 90°C aerated MEA solution of pH 9 at applied potentials in both the active–passive transition and the transpassive regions. The mechanism of cracking was attributed to stress assisted dissolution of copper from the alloy matrix.